Protein kinases are a large class of enzymes which catalyze the transfer of the γ-phosphate from ATP to the hydroxyl group on the side chain of Ser/Thr or Tyr in proteins and peptides and are intimately involved in the control of various important cell functions, perhaps most notably: signal transduction, differentiation and proliferation. There are estimated to be about 2,000 distinct protein kinases in the human body (Hunter, 1987, 1994, Hanks & Hunter, 1995), and although each of these phosphorylate particular protein/peptide substrates, they all bind the same second substrate ATP in a highly conserved pocket.
Inhibitors of various known protein kinases could have a variety of therapeutic applications provided sufficient selectivity, and acceptable in vivo pharmacological properties, can be incorporated into such inhibitors (Levitzki, 1996a). Perhaps the most promising potential therapeutic use for protein kinase inhibitors is as anti-cancer agents. This potential application for protein tyrosine kinase (“PTK”) inhibitors has been highlighted in many recent reviews (e.g. Lawrence & Hiu, 1998, Kolibaba & Druker, 1997, Showalter & Kraker, 1997, Patrick & Heimbrook, 1996, Groundwater et al., 1996, Levitzki, 1995). The foundation for this application is based partly upon the fact that about 50% of the known oncogene products are PTKs and their kinase activity has been shown to lead to cell transformation (Yamamoto, 1993).
The PTKs can be classified into two categories (Courtneidge, 1994), the membrane receptor PTKs (e.g. growth factor receptor PTKs) and the non-receptor PTKs (e.g. the src family of proto-oncogene products). There are at least 9 members of the src family of non-receptor PTK's with pp60c-src (hereafter referred to simply as “src”) being the prototype PTK of the family wherein the ca. 300 amino acid catalytic domains are highly conserved (Rudd et al., 1993, Courtneidge, 1994). The hyperactivation of src has been reported in and number of human cancers, including those of the colon (Mao et al., 1997, Talamonti et al., 1993), breast (Luttrell et al., 1994), lung (Mazurenko et a, 1992), bladder (Fanning et al., 1992) and skin (Barnekow et al., 1987) as well as in gastric cancer (Takeshima et al., 1991), hairy cell leukemia (Lynch et al., 1993) and neuroblastoma (Bjelfman et al., 1990). Overstimulated cell proliferation signals from transmembrane receptors (e.g. EGFR and p185HER2/Neu) to the cell interior also appears to pass through src (Mao et al., 1997, Parsons & Parsons, 1997, Bjorge et al., 1996, Taylor & Shalloway, 1996). Consequently, it has recently been proposed that src is a universal target for cancer therapy (Levitzki, 1996) because its' hyperactivation (without mutation) is involved in tumor initiation, progression and metastasis for many important human tumor types.
In view of the large, and growing, potential for inhibitors of various protein kinases, a variety of approaches to obtaining useful inhibitors is needed. The status of the discovery of PTK inhibitors (Lawrence & Niu, 1988, Showalter & Kraker, 1997, Patrick & Heimbrook, 1996, Groundwater et al., 1996, Budde et al., 1995, Levitzki & Gazit, 1995) has been extensively reviewed. Random screening efforts have been successful in identifying non-peptide protein kinase inhibitors but the vast majority of these bind in the highly conserved ATP binding site. A notable recent example of such non-peptide, ATP-competitive, inhibitors are the 4-anilinoquinazolines, and analogs, which were shown to be effective against the epidermal growth factor receptor PTK (EGFRTK) (e.g. Rewcastle et al., 1996). Although this class of inhibitors was reported to be selective for the EGFR PTK vs. six other PTKs (including src, Fry et al., 1994) it is unknown what their effect is on most of the remaining 2,000 protein kinases that all bind ATP as well as a large number of other ATP, ADP, GTP, GDP, etc. utilizing proteins in the body. Therefore, potential side effects from PTK inhibitor drugs that mimic ATP, which might only be discovered after expensive animal toxicity studies or human clinical trials, are still a serious concern. Also, although this class of compounds was a nice discovery and is undergoing further exploration, they do not provide a rational and general solution to obtaining non-peptide inhibitors for any desired PTK, e.g. in this case src. The risk of insufficient specificity in vivo with ATP-competitive PTK inhibitors has also been noted by others, along with the inherent three order of magnitude reduction in potency these inhibitors display when competing with the mM levels of intracellular ATP rather than the μM levels used in the isolated enzyme assays (e.g. see Lawrence & Niu, 1998, Hanke et al., 1996, Kelloff et al., 1996).
An older, and more extensively studied, class of non-peptide PTK inhibitors is erbstatin and the related tyrphostins (see reviews). This class of inhibitors are active against the receptor PTKs and their mode of inhibition is complex but does not appear to involve binding in the peptide substrate specificity site regions of the active site (Hsu et al., 1992, Posner et al., 1994). Furthermore, they are inactive against the isolated PTK when the unnatural assay metal Mn2+ is replaced with the natural Mg2+ (Hsu et al., 1992), are chemically unstable (Budde et al., 1995, Ramdas et al., 1995 & 1994), and are know to be cytotoxic to normal and neoplastic cells by crosslinking proteins (Stanwell et al., 1995 & 1996) as well as inhibit cell growth by disrupting mitochondria rather than PTK inhibition (Burger et al., 1995).
An important contribution to the protein kinase field has been the x-ray structural work with the serine kinase cAMP-dependent protein kinase (“PKA”) bound to the 20-residue peptide derived from the heat stable inhibitor protein, PKI(5–24), and Mg2ATP (Taylor et al., 1993). This structural work is particularly valuable because PKA is considered to be a prototype for the entire family of protein kinases since they have evolved from a single ancestral protein kinase. Sequence alignments of PKA with other serine and tyrosine kinases have identified a conserved catalytic core of about 260 residues and 11 highly conserved residues within this core (Taylor et al., 1993). Two highly conserved residues of particular note for the work proposed herein are the general base Asp-166 which is proposed to interact with the substrate OH and the positively charged residue, Lys-168 for serine kinases and an Arg for tyrosine kinases (Knighton et al., 1993), which is proposed to interact with the γ-phosphate of ATP to help catalyze transfer of this phosphate. Two additional important PKA crystal structures have been reported (Madhusudan et al., 1994), one for the ternary PKA:ADP:PKI(5–24) complex wherein the PKI Ala 21 has been replaced with Ser (thereby becomming a substrate), and one for the binary PKA:PKI(5–24) complex wherein the PKI Ala 21 has been replaced with phosphoserine (an end product inhibitor). The ternary complex shows the serine OH donating a H-bond to Asp-166 and accepting a H-bond from the side chain of Lys 168. The binary complex shows the phosphate group of phosphoserine forming a salt bridge with the Lys-168 side chain and within H-bonding distance of the Asp-166 carboxyl group. These structures support the earlier proposed roles for Asp-166 and Lys-168 in the catalytic mechanism.
The x-ray structures of PKA show that the enzyme consists of two lobes wherein the smaller lobe binds ATP and the larger lobe the peptide substrate. Catalysis occurs at the cleft between the lobes. The crystallographic and solution structural studies with PKA have indicated that the enzyme undergoes major conformational changes from an “open” form to the “closed” catalytically active form as it binds the substrates (Cox et al., 1994). These conformational changes are presumed to involve the closing of the cleft between the two lobes as the substrates bind bringing the γ-phosphate of ATP and the Ser OH in closer proximity for direct transfer of the phosphate.
However, the inhibitors of protein kinases still lack the specificity and potency desired for therapeutic use. Due to the key roles played by protein kinases in a number of different diseases, including cancer, psoriasis, arthrosclerosis, and their role in regulating immune system activity, inhibitors of specific protein kinases are needed. The present invention provides a novel approach for designing protein kinase inhibitors, which are more potent as well as being more specific for the targeted pathways.